1. Technical Field
The present invention generally relates to wireless communication networks, and particularly relates to a method and system for determining cell identification in a multi-cell network.
2. Background
In comparison to existing wireless communication networks, such as those based on GSM and current iterations of Wideband CDMA (WCDMA), developing and planned wireless communication networks will offer comparatively high data rates through the use of increasingly sophisticated air interfaces. These changes include the likely adoption of newer transmission techniques, such as Orthogonal Frequency Division Multiplexing (OFDM), which already finds use, for example, in the 802.11 Wireless LAN standards.
One particular proposal represents an extension of the Universal Mobile Telecommunications System (UMTS) and is specified in “Release 8” of the Third Generation Partnership Project (3GPP). The Release 8 proposal is referred to as Long Term Evolution (“LTE”), and is also referred to as “Evolved UTRA” (Universal Terrestrial Radio Access) and “Evolved UTRAN” (Universal Terrestrial Radio Access Network).
LTE can be seen as an evolution of the 3G WCDMA standard, although LTE systems will use OFDM as a multiple access technique (referred to as “OFDMA”) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, LTE will support data rates up to 100 Mb/s, at least for the largest bandwidth allocations. Additionally, not only high rate services are expected to use LTE, but also low rate services like voice.
Mobility management represents an important aspect of LTE. As user equipment (UE) moves within an LTE coverage area for example, the use of synchronization signal transmissions and cell search procedures provide a basis for the UE to detect and synchronize with individual cells. More generally, to communicate with a particular cell, UEs must be able to determine one or more transmission parameters used by the cell. Such information is obtained by decoding the cell's Broadcast Channel (BCH) signal, which in turn first requires the UE to synchronize to the cell's symbol and radio timing, and determine the cell's identifier (cell ID).
To that end, each cell transmits a Primary Synchronization Signal (P-SyS) and Secondary Synchronization Signal (S-SyS) on a per 5 ms basis. These signals allow the UE to synchronize to the symbol/frame timing of any given cell's transmissions, and thereby receive the cell's reference signal, from which the UE detects the cell's reference symbol sequence and thereby obtains the cell ID. More particularly, the P-SyS, which is based on Zadhoff-Chu sequences, is used for channel estimation by the UE and the channel estimates are then used for decoding the S-SyS, which provides frame boundary synchronization and cell group information.
Cell synchronization enabled by the P-SyS and S-SyS thus allows the UE to acquire the cell's reference symbol sequence, which identifies the cell. Reference symbol sequence generation relies on the formation of symbol-by-symbol products of one of three orthogonal sequences and one of 170 pseudorandom sequences. That arrangement yields 510 unique reference symbol sequences, thus providing for 510 unique cell identifications.
The document R1-062990 (2006) produced by TSG-RAN WG1 #46bis outlines a basic cell search procedure for UEs operating in an LTE network where good channel estimation is important for successful reference symbol sequence reception. The proposed cell search proposal involves four basic steps. First, the UE identifies a 5 ms synchronization timing from the P-SyS transmissions. For example, the UE determines a 5 ms synchronization timing for the strongest of one or more received P-SyS signals.
Second, the UE determines radio timing from the S-SyS associated with the (still unidentified) cell of interest offering the best P-SyS. Here, determining radio timing denotes the determination of the full 10 ms frame synchronization, remembering that S-SyS yields frame boundary timing, which allows the UE to synchronize with the frame timing of the cell of interest.
The proposed processing continues with using the cell-group scrambling code, determined from the S-SyS, to determine the reference symbol sequences being transmitted by the cells in the group. That determination allows the UE to determine cell ID for decoding the BCH from the cell of interest. More particularly, the UE estimates the channel transfer functions or impulse responses based on P-SyS and/or S-SyS; uses channel estimates to mitigate the impact of the propagation channels on the received signal; removes the scrambling code effects; and correlates with all possible reference symbol sequences to identify the sequence (and thus cell ID) that delivers the maximum correlation peak.
Problematically, however, without further narrowing information, the search space associated with determination of the reference symbol sequences is undesirably large—i.e., an undesirable number of hypotheses are involved in the correlation processing for reference symbol sequence identification. Additionally, reference symbol sequence determination in this context is compromised by channel estimation inaccuracies.